At low frequencies (i.e., f.ltoreq.2MHz) one can easily achieve a high magnetic coupling between windings due to a common availability of magnetic cores that feature a high magnetic permeability (i.e., .mu..gtoreq.R.B.S. 5000) and relatively low core losses. Since magnetic coupling determines how well the magnetic field is confined to the core, it is evident that good magnetic coupling results in less magnetic leakage and better reproduction of the signal at the secondary. As the signal frequency increases, the permeability decreases and the core losses increase thereby contributing to increased magnetic leakage. The increased magnetic leakage causes signal distortion. Further, as signal frequency increases, transformer stray parameters play increasingly significant roles in limiting good performance.
One solution to the referenced problems is to use a special high frequency core where the core losses at high frequencies are relatively low and the magnetic permeability .mu. is quite flat with frequency. However, the value of .mu., as compared to low frequency cores, is highly reduced (.mu..ltoreq.1000). As a result, the use of the special high frequency transformer cores require some extra effort to compensate for the reduced magnetic coupling. One prior art solution is to place primary and secondary windings very close together by using a parallel bonded or twisted wire pair (FIG. 1). Unfortunately, by placing the windings close together, the interwinding capacitance gets large and exceeds, by many times, the winding shunt distributed capacitance. However, when the transformation ratio is 1:1 and there is no ground phase reversal, one can show that there will be almost no varying electric field between the primary and secondary. Under these circumstances, high interwinding capacitance resulting from high electrical coupling will have virtually no effect on frequency performance of the 1:1 transformer. However, even here, if one is not careful in placing grounds, the interwinding capacitance can add to the shunt capacitance and the performance advantage to be gained by the good electrical coupling is completely lost.
In view of the above, transformers with transformation ratios other than 1:1 are rarely used at high frequencies if signal distortion is of concern. Placing coacting or commonly interacting primary and secondary windings at a distance from one another on a single magnetic core to reduce the interwinding capacitance will significantly increase the magnetic leakage. Placing unbalanced windings close together will result in a highly varying interwinding electrical field. In both cases, high frequency performance will be degraded.